There are various uses for organopolysiloxanes containing SiH groups as starting materials in silicone chemistry. It is therefore of particular interest to be able to synthesize these compounds economically, in the simplest way possible and in high yields from easily accessible starting materials. As starting materials, particularly the easily accessible, appropriate organopolysiloxanes containing silicon halide groups come into consideration. The conversion of chlorosiloxanes into the corresponding hydrogensiloxanes and similar reactions with chlorosilanes are known from the literature.
For example, a method is disclosed in U.S. Pat. No. 3,099,672 for reacting halogensilanes or alkoxysilanes with sodium hydride at a temperature of 175.degree. to 350.degree. C. For this method, which can be employed only with silanes and not with organopolysiloxanes, a hydrogen atom takes the place of the halogen or alkoxy group, and the yields from this reaction vary.
Pursuant to the U.S. Pat. No. 3,099,672, the sodium hydride is used as such, as a suspension in mineral oil or other high-boiling, inert hydrocarbons or in the form of a coating on sodium chloride particles. The need to use relatively high temperatures makes the method more expensive and frequently leads to unwanted by-products.
The U.S. Pat. No. 3,535,092 discloses a method for converting compounds, which have silicon halide groups, with sodium hydride. Said method proceeds readily at room temperatures or only moderately elevated temperatures. This is made possible through the use of special solvents, such as hexaalkylphosphoramides, octaalkylpyrophosphoramides and tetraalkylurea. The alkyl groups in the solvent contain 1 to 4 carbon atoms. It can be inferred from the U.S. Pat. No. 3,535,092 that these solvents are to have a catalytic effect. The method can be used with halogensilanes as well as with halogensiloxanes. Alkoxysilanes do not react under the same conditions. Since the solvents have high boiling points, the products, which contain SiH groups, are distilled from the reaction mixture. By these means, the usefulness of this method is restricted to those silanes or siloxanes containing SiH groups, which can be distilled off at a justifiable cost and without thermal decomposition. Removal of the aforementioned solvents by washing is not possible without further ado. When the solvent is removed with water, sodium hydroxide is formed from the excess sodium hydride and leads to the formation of SiOSi bonds, hydrogen being split off in the process. However, since the solvents named in this patent are physiologically questionable, a complete separation of the silicon-containing products from the solvents is a necessary prerequisite for most applications.
Instead of the sodium hydride, complex hydrides, such as sodium aluminum hydride, have also already been used for exchanging silicon halide groups for silicon hydride groups. For example, the German Auslegeschrift 10 85 875 discloses a method for the preparation of polysiloxane hydrides of the general formula ##STR2## wherein n is a whole number, particularly from 1 to 5 and R represents hydrogen or alkyl and aryl groups, which is characterized in that polysiloxanes of the general formula ##STR3## in which X represents chlorine or alkyl and aryl groups and n has the meaning given above, are reacted with lithium aluminum hydride. According to the examples of the German Auslegeschrift 10 85 875, 1,7-dichlorooctamethyltetrasiloxane is converted with lithium aluminum hydride in tetrahydrofuran in a 74% yield into 1,7-dihydrooctamethyltetrasiloxane. The conversion of 1-chloropentamethyldisiloxane into 1-hydropentamethyldisiloxane with lithium aluminum hydride takes place in a 57% yield.
As is evident from the "Polish Journal of Chemistry", 53, (1979), 1383-1386, by repeating the method of the German Auslegeschrift 10 85 875, SiOSi bonds are split in competition with the exchange of the silicon halide groups, so that silanes, such as the gaseous and easily ignitable dimethylsilane, are formed in considerable amounts as by-products. This is in agreement with the information in "Chemie und Technologie der Silicone" by W. Noll, Verlag Chemie, 1968, page 206, where it is stated that, due to the action of lithium aluminum hydride in ether on linear polydimethysiloxanes, siloxane bonds are converted into aluminosiloxane bonds and, at the same time, silanes are split off. In the case of silicate esters, the splitting reaction proceeds practically quantitatively. This splitting of SiOSi bonds by the action of lithium aluminum hydride can also be inferred from the Zeitschrift fuer Naturforschung, 10b, (1955), 423 to 424.
The German Auslegeschrift 15 68 255 discloses a similar method, in which trisodium aluminum hexahydride is used as complex aluminum hydride. The splitting of SiOSi groups is a competing reaction also when this complex hydride is used.
The German Auslegeschrift 10 28 575 starts out from the assumption that the hydrogenation with LiH is very much slower than that with LiAlH.sub.4. One would have to work with a large excess of LiH in order to have bearable yields. Large amounts of solvents (72.5 kg of ether for 1 kg of SiH.sub.4) are required for working with LiAlH.sub.4. Said Auslegeschrift teaches instead that the reaction with hydrogen can be carried out in the presence of sodium or sodium hydride under pressure and at an elevated temperature. As shown by the Examples, the yields are of the order of 80 to 90%. However, when sodium is used, the Wurtz reaction (--Si--Cl+2 Na+Cl--Si-- .fwdarw. --Si--Si--), which is to be avoided, occurs as a side reaction, particularly at the given high temperatures.
From the German patent 36 37 273, a method is known for preparing organopolysiloxanes containing SiH groups. For this method, halogensiloxanes are reacted with LiH, NaH, KH, CaH.sub.2 or MgH.sub.2, using ether as reaction medium and with continuous removal of the metal halide deposited on the surface of the metal hydride particles during the reaction by the action of mechanical energy or ultrasound so as to form a fresh surface. From the point of view of good value, the safety and the reducing power, MgH.sub.2, in particular, is utilized since the alkali hydrides are expensive and hazardous to handle, whereas the reducing power of CaH.sub.2 is too low. During the reaction with MgH.sub.2, however, which is to be preferred, easily inflammable portions of dimethylsilane are formed in a side reaction. On the one hand, said dimethylsilane reduces the SiH yield of the process. On the other, it leads indirectly to a change in the original chain distribution, particularly if the equilibration of the chlorosilane has been carried out with FeCl.sub.3.
The European patent 0 476 597 teaches the preparation of 1,1,3,3-tetramethyl-1,3-disiloxane by reducing 1,1,3,3-tetramethyl-1,3-dichloro-1,3-disiloxane with a metal hydride, such as LiAlH.sub.4, NaAlH.sub.4, LiBH.sub.4, LiH or NaH in an organic solvent under a blanket of an inert gas at temperatures of 0.degree. to 50.degree. C. LiH and NaH, which are not complex hydrides, do not give acceptable yields without grinding. On the other hand, the complex hydrides, LiAlH.sub.4, NaAlH.sub.4 and LiBH.sub.4 are very expensive and, in the case of LiAlH.sub.4 and NaAlH.sub.4, dangerous to handle.